It is well known in the art that the resistance modulation of Hall elements or magnetoresistors can be employed in position and speed sensors with respect to moving ferromagnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467; 4,926,122; and 4,939,456).
Increasingly more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing not only precise position information, but also, during cranking, a direction of rotation signal. Currently, as shown in FIG. 1, direction of rotation, clockwise (CW) 32 or counterclockwise (CCW) 34, is most commonly obtained from an encoder wheel 10, also called a target wheel, having a plurality of teeth 12 and slots 14 of essentially equal length which have a spacing between adjacent teeth or a spacing between adjacent slots of P, called the tooth/slot pitch. The teeth 12 and slots 14 are sensed by SENSOR116 and SENSOR218 in quadrature (i.e. with spacing between sensors equal to one-quarter of the tooth/slot pitch P). Equal lengths of teeth 12 and equal lengths of slots 14 and the one-quarter pitch spacing between sensors are preferred, but not absolutely necessary. The square wave output signals from SENSOR116 and SENSOR218 are fed as a clock input 20 and a data input 22 into a D type FLIP-FLOP 24 whose output 26 provides a direction indication and is either high (H) 28 or low (L) 30 depending on the direction of rotation of the target wheel 10. Due to the large P/4 distance between SENSOR116 and SENSOR218, a major disadvantage of the current method is the inability to detect the change of rotational direction of the target wheel when a single tooth edge, such as tooth edge 36, is oscillating around a single sensor, such as SENSOR116. In this case, SENSOR116 would detect the back and forth oscillating single tooth edge 36 as the rising and falling edges of passing teeth 12. Since the direction indication 26 would remain unchanged, serious position errors would be generated. For this reason, and due to the necessity of adding a second sensor, the conventional method of detecting direction of rotation is not suitable for automotive crankshaft sensors.
What is needed is a method and apparatus whereby the direction of rotation of an automotive crankshaft can be obtained from the passage of a single tooth edge of a target wheel.